The present invention relates generally to a subsea well intervention system, and more specifically to a riserless modular subsea well intervention system.
Oil and gas wells frequently require subsurface maintenance and remediation to maintain adequate flow or production. This activity is commonly referred to as “workover.” During the workover specialized tools are lowered into the well by means of a wire line and winch. This wire line winch is typically positioned on the surface and the workover tool is lowered into the well through a lubricator and blowout preventer (BOP). Workover operations on subsea wells require specialized intervention equipment to pass through the water column and to gain access to the well. The system of valves on the wellhead is commonly referred to as the “tree” and the intervention equipment is attached to the tree with a BOP.
The commonly used method for accessing a subsea well first requires installation of a BOP with a pre-attached running tool for guiding the BOP to correctly align and interface with the tree. The BOP/running tool is lowered from a derrick that is mounted on a surface vessel such as a drill ship or semi-submersible platform. The BOP/running tool is lowered on a segmented length of pipe called a “workover string”. The BOP/running tool is lowered by adding sections of pipe to the workover string until the BOP/running tool is sufficiently deep to allow landing on the tree. After the BOP is attached to the tree, the workover tool is lowered into the well through a lubricator mounted on the top of the workover string. The lubricator provides a sealing system at the entrance of the wire line that maintains the pressure and fluids inside the well and the workover string. The main disadvantage of this method is the large, specialized vessel that is required to deploy the workover string and the workover string needed to deploy the BOP.
Another common method for well intervention involves the use of a remotely operated vehicle (ROV) and a subsea lubricator to eliminate the need for the workover string and therefore the need for a large, specialized vessel. Current state of the art methods require that the BOP and lubricator are assembled on the surface and then lowered to the seafloor with winches. When the BOP is in the vicinity of the tree, the ROV is used to guide the BOP/lubricator package into position and lock it to the tree. A control umbilical, attached to the BOP/lubricator package is then used to operate the various functions required to access the well. The workover tool can then be lowered on a wire line winch and the ROV is utilized to install the tool in the lubricator so that workover operations can be accomplished. The umbilical provides control functions for the BOP as well as a conduit for fluids circulated in the lubricator.
A common problem with both the workover string method and the BOP/lubricator package method is encountered during a “drive-off” condition. A drive-off condition occurs when by accident or design the surface vessel is forced to move away from its position over the well without first recovering the equipment attached to the tree. Vessels in deep water are commonly held in position over the well by computer controlled, dynamic thrusters. If for any reason, there is a failure in the computer, the thrusters, or any related equipment, the vessel will not be able to hold position or it may be driven off position by incorrect action of the thrusters. In the event of a drive-off condition, the operator must close the valves on the tree and release the BOP so that the intervention equipment can be pulled free of the well. With the drill string method, the BOP is supported by the drill string. With the BOP/Lubricator method, the equipment must be lifted by the surface winches that must be kept continuously attached to the BOP/lubricator equipment. In either case, large pieces of equipment remain hanging below the vessel until they can be recovered.
What is needed is a method and apparatus for the installation of subsea well intervention equipment that eliminates the need to recover the equipment in a drive-off condition.